A typical electrostatographic printing machine (such as a photocopier, laser printer, facsimile machine or the like) employs an imaging member that is exposed to an image to be printed. Exposure of the imaging member records an electrostatic latent image on it corresponding to the informational areas contained within the image to be printed. The latent image is developed by bringing a developer material into contact therewith. The developed image recorded on the photoconductive member is transferred to a support material such as paper either directly or via an intermediate transport member. The developed image on the support material is generally subjected to heat and/or pressure to permanently fuse it thereto.
Two types of developer materials are typically employed in electrostatographic printing machines. One type of developer material is known as a dry developer material and comprises toner particles or carrier granules having toner particles adhering triboelectrically thereto. Another type of developer material is a liquid developer material comprising a liquid carrier or dispersant having toner particles dispersed therein.
Development with liquid developers in full color imaging processes has many advantages, such as a texturally attractive print because there is substantially no toner height build-up, whereas full color images developed with dry toners often exhibit height build-up of the image where color areas overlap. Further, full color prints made with liquid developers can be made to have either a uniformly glossy or a uniformly matte finish, whereas uniformity of finish is difficult to achieve with powder toners because of variations in the toner pile height.
High toner pile height is a major document appearance problem for powder xerography. It is obvious to the customer not only as increased document thickness but also in other undesirable ways, such as paper curl. In addition to being an aesthetic dissatisfier, paper distortion due to curl and ripple increases the jam rate and complicates paper handling and document finishing. This is objectionable in any market, but especially in the production color printing market, which demands high-speed reliable operation and is accustomed to the look and feel of lithography.
Toner pile height can be reduced by reducing toner size, but the performance of current xerographic subsystem designs would be compromised for average particle sizes less than about 5 .mu.m. On the other hand, in conventional systems, if toner mass is reduced without reducing toner size, the toner does not completely cover the paper even in the Dmax areas. Incomplete paper coverage leads to significant color and image quality degradation, since even a small amount of white light from bare paper can reduce image chroma noticeably. This is particularly severe for high-chroma and/or low-lightness colors, such as deep blue.
A need exists for an electrostatic printing machine that can produce texturally attractive color prints with substantially no height build-up employing dry developers. A simple, relatively inexpensive, and accurate approach to produce color prints in such printing systems has been a goal in the design, manufacture and use of electrophotographic printers. This need has been particularly recognized in the process color and highlight color portion of electrophotography. The need to provide accurate and inexpensive color reproduction with dry developers has become more acute, as the demand for high quality, relatively inexpensive color images and the machines that produce them have increased.
The present invention obviates the problems noted above by utilizing an apparatus for forming an image on a recording sheet having low toner pile height. A latent image is formed on an imaging member. A developer unit develops the latent image with a less than monolayer of toner particles on the imaging member. The developed image is transferred to a compliant intermediate member whereupon a filming station spreads the toner particles to form a film layer from the toner particles. The station includes an optional heater for heating said monolayer of toner particles to a temperature sufficient to cause the toner particles present on the intermediate member to soften; and a heated roller for spreading said monolayer of toner particles to generate said film layer. The above process is repeated for subsequent colored toners to produce a multi-film layer color image. Thereafter, the multi-film layer of toner material is transfused from said intermediate member onto a recording sheet such as paper.